This invention relates to generation of sawtooth waveforms, and in particular, to a sawtooth waveform generator adapted to provide a train of sawtooth waveform signals, having equal bipolar excursions of predetermined amplitude and of a frequency having a predetermined relationship with the frequency of an external AC input signal.
Sawtooth waveforms are commonly utilized for modulation and other applications requiring that the sawtooth have precisely defined characteristics, such as equal bipolar excursions of a predetermined magnitude and a time duration having a predetermined relationship with another signal, such as for example, an alternating current signal. For example, sawtooth waveforms are utilized to convert variable amplitude signals into pulse width modulated signals in connection with AC power conversion.
In electric power conversion systems an AC input signal is commonly coupled through a static switching arrangement, such as back to back connected silicon controlled rectifiers. Firing pulses are supplied to the switching arrangement to control its conduction period during each cycle of the AC input signal. The conduction period is a function of an analog control signal, such as the amplitude of a DC signal. Control systems for this purpose commonly modulate the analog signal with a train of sawtooth waveforms to provide pulse width modulated pulses, i.e. a train of pulses having a time duration representative of the desired conduction period of the switching arrangement. The sawtooth waveforms utilized in certain control applications must have equal bipolar excursions of predetermined amplitude and a time duration, i.e. frequency, which is related in a predetermined manner with the frequency of the AC input signal. Commonly, the frequency of the sawtooth waveform is twice the frequency of the AC input, thus permitting modulation during each half cycle of the AC input.
Sawtooth generators in order to provide the required type of sawtooth must be adjusted from time to time to maintain the requisite equal bipolar excursion of predetermined amplitude and the required frequency relationship. Such adjustment is required because of tolerance variations of the components of the generator, and because of changes of the frequency of the AC input, e.g. 50 cycle vs. 60 cycle operation.